It is not clear what Trump means by a "constitutional crisis" but he suggested Monday to supporters in Michigan that Clinton could face a lengthy criminal investigation and possible a criminal trial as a sitting president as the FBI continues to examine her email practices.

There is no agreed upon definition of the term, according to Stanford Law Professor Pamela Karlan, who specializes in constitutional law.

"Constitutional crisis is one of those phrases like 'judicial activism' that has a gazillion different meanings to different people," she said. "I think it's something where the Constitution literally can't operate or there's nothing in the Constitution to provide for how things will operate."

It has been an open question, for example, under what, if any, circumstances a sitting president can be prosecuted while in office. The Justice Department's office of legal counsel said in 1973 that criminally prosecuting a president would be unconstitutional because it would undermine the executive branch. A 2000 memo from the same office reached a similar conclusion. But the issue has never been tested in court.

Sitting presidents can face civil lawsuits. Jim Jacobs, a professor of criminal law at New York University School of Law, noted that Bill Clinton faced a civil lawsuit for sexual harassment during his presidency that led to an impeachment proceeding. The Senate ultimately acquitted him and "the government went on," Jacobs said.

At the moment, there's no reason to think Clinton is in danger of being criminally prosecuted, he said. "She was cleared the last we heard from the FBI and we don't know where this latest investigation will lead."

Perhaps the nation's greatest constitutional crisis occurred during Watergate when President Richard Nixon refused to turn over White House audiotapes to Archibald Cox, a special prosecutor investigating the scandal.

Cox was then fired at Nixon's request by then-Solicitor General Robert Bork. This incident became known as "the Saturday Night Massacre." The firing was up to Bork only because Attorney General Elliot Richardson and Deputy Attorney General William Ruckelshaus both resigned rather than follow Nixon's order.

Nixon later resigned as he faced the likelihood of being impeached, and was eventually pardoned by President Gerald Ford.